1. Field of the Invention
The present invention relates generally to gas turbine engines and, more particularly, to a blade containment system and method for use in gas turbine engines.
2. Related Art
Conventional high bypass ratio turbofan engines, which are included in the more general category of gas turbine engines and which may be used for aircraft propulsion, typically include a fan, booster, high pressure compressor, combustor, high pressure turbine, and low pressure turbine in serial axial flow relationship about a longitudinal centerline axis of the engine. The high pressure turbine is drivingly connected to the high pressure compressor via a first rotor shaft, and the low pressure turbine is drivingly connected to both the fan and booster via a second rotor shaft. The fan includes an annular disk and a plurality of radially extending blades mounted on the disk, wherein the disk and the blades are rotatable about the longitudinal centerline of the engine. The engine further includes an annular casing which surrounds the blades in radially spaced relationship. Furthermore, in order to attenuate noise emanating from the engine, the engine typically includes a plurality of circumferentially segmented acoustic panels which are disposed about the interior surface of the annular casing in a location which is axially forward of the blades. The acoustic panels are typically made of a relatively light construction which includes a perforated composite facesheet adjacent the engine flowpath, a honeycomb structure bonded to the facesheet wherein the honeycomb structure is typically sub-divided into two portions separated by a septum, and a radially outer composite backsheet which is bonded to the honeycomb structure.
Gas turbine engines typically operate at relatively high rotational speeds and, for high bypass ratio turbofans the fan blades typically include a relatively large radially extending length. Consequently, the fan blades possess relatively high kinetic energy even when the blades are fabricated using lightweight alloys or composite materials. Containment of fan blades has been a continuing problem for the industry and it has been particularly problematic in the area of the engine immediately forward of the blades for the following reasons. During engine operation various known events may occur, such as blade contact with foreign or domestic objects, which may cause a blade to be released from the fan disk. The first released blade typically contacts the outer containment structure located directly outward of the blades in such a manner that the impact load from the blade is distributed over an area of the casing corresponding to the full chord length of the blade tip. However, in a known scenario the platform portion of the first released blade contacts a trailing blade in the area between the midspan shroud and the platform portion of the trail blade such that approximately the outer two-thirds of the trail blade is severed and recoils forward such that the trailing edge comer of the tip of the trail blade impacts the acoustic panels positioned forward of the blades. The trailing edge tip comer of the trail blade may then effectively act as a cutting tool resulting in significant damage to not only the acoustic panels but to the surrounding containment structure comprising the annular casing as well. Various devices have been used previously to provide containment of fan blades in general, and to provide containment of fan blades in the area forward of the fan blades in particular.
One prior method of providing containment of fan blades in the area of the engine forward of the fan blades may be referred to as "static containment", wherein the annular fan casing is manufactured from a high strength material having an adequate radially extending shell thickness to absorb the kinetic energy or the impacting fan blade and thereby contain the blade. While the "static containment" system has proven to be an effective means for containing fan blades in the region of the engine forward of the fan blades, it is not weight efficient.
Another prior method of containing blades of turbomachinery, at least with respect to the area of the engine immediately outward of the rotating blades, is disclosed in U.S. Pat. No. 4,149,824 to Adamson which is assigned to the assignee of the present invention. Adamson discloses a high-strength annular ring which is supported in radially spaced relationship over a stage of rotating blades, wherein the annular ring is supported within a support structure in such a manner that when the ring is impacted or contacted by a blade with a predetermined impact force, energy associated with the blade is absorbed by imparting spin to the annular ring with respect to the stationary support structure. While the Adamson device is effective in certain applications, it has its limitations. Any dynamic containment device employing an annular ring, such as that disclosed in Adamson, does not lend itself to field retrofit applications wherein the fan casing is manufactured by a process, such as an extrusion process, which produces relatively large diametral tolerances associated with the interior surface of the fan casing. The use of an annular ring in such a field retrofit application would either require machining of the interior surface of the fan casing which would be very expensive, or, if the casing was not machined could result in excessive radial clearances between the annular ring and the fan casing which would not be structurally sound and could in turn cause a fan rotor vibratory response resulting in premature blade failure.
In view of the foregoing, prior to this invention a need existed for an improved blade containment system to resolve the problems associated with prior containment systems and thereby provide increased capability for containment of blades in a cost and weight efficient manner.